How to calculate square footage: in short
For a rectangular room, multiply the length by the width in feet: a 12 by 14 foot room is 168 square feet. For other shapes, split the room into rectangles and triangles, work out each part and add them up.
The square footage formula
Area = length × width, with both sizes in feet. If a size has inches, turn the inches into a decimal first: 6 inches is 0.5 ft, so 12′ 6″ is 12.5 ft.
- 12 × 14 = 168 sq ft
- 12.5 × 10 = 125 sq ft
- To square meters, multiply by 0.0929: 168 sq ft is 15.6 m²
L-shaped rooms
Take the full rectangle around the room and subtract the missing corner. Or split the L into two rectangles and add them. Both give the same answer.
Example: the full rectangle is 20 × 15 = 300 sq ft and the missing corner is 8 × 6 = 48 sq ft, so the room is 300 − 48 = 252 sq ft. Split into two rectangles: 20 × 9 = 180 and 12 × 6 = 72, together 252 sq ft.
Triangles and walls at an angle
A triangle is half of base × height. A triangle with a 10 ft base and a height of 6 ft is 30 sq ft. Where a wall runs at an angle, split the room into a rectangle and a triangle and add them.
How do you estimate square footage?
For a quick estimate of a house, measure the outside walls of each floor, multiply length by width and add the floors. Leave out the garage and anything below ground. For a room, measure the two longest sides wall to wall.
An estimate from the outside walls is larger than the sum of the rooms, because it includes the walls. For flooring or paint, use the inside sizes; for a listing, the outside.
Several rooms and a whole house
Work out each room and add them together. For the official square footage of a house, US appraisers follow ANSI Z765: they measure the outside of the exterior walls and count only finished space above ground.
Adding up rooms measured from the inside gives a smaller number, because the walls are left out. Both numbers are right for their own purpose: inside sizes for flooring and furniture, outside sizes for the listing.
What counts and what does not (ANSI Z765)
| Space | In the square footage of a house? |
|---|---|
| Finished rooms above ground, with closets and hallways | Yes, this is the gross living area |
| A floor with any part below ground (basement) | No, listed separately as below-grade area |
| Garage | No, listed separately |
| Porch, deck, unfinished attic | No |
| Parts of a room under a sloped ceiling lower than 5 ft | No |
Common mistakes
- Reading 12′ 6″ as 12.6 ft. Six inches is half a foot, so it is 12.5 ft.
- Rounding every room before adding. Round only the total.
- Forgetting closets and the bump of a bay window.
- Counting the garage or a finished basement as living area.
- Comparing an inside measurement with a listing that measured the outside walls.
Figure 1. One floor of 25 × 17 ft. The rooms add up to 425 sq ft.
Sources
- ANSI Z765-2021, Square Footage: Method for Calculating, Home Innovation Research Labs · Checked against the source on September 24, 2026
- ANSI Z765 frequently asked questions, Home Innovation Research Labs · Checked against the source on September 24, 2026
About this page · Written by the Squaremetry team, based on ANSI Z765-2021, Square Footage: Method for Calculating and ANSI Z765 frequently asked questions. Last checked on October 1, 2026. Found a mistake? Send us a message.